Washington, D.C. is served by three major airports.
Ronald Reagan Washington National Airport (DCA), is the closest and most convenient airport to D.C., located three miles to the south in Arlington, Virginia, just across the Potomac River.
Washington Dulles International Airport (IAD) is located 26 miles west of D.C. in Dulles, Virginia. It is an architectural masterpiece, but unfortunately some functionality was scrapped in pursuit of aesthetics—you will have to take a train between the main building and the concourses. If you have extra time, consider taking the VRTA Air and Space Museum Shuttle to see the Smithsonian Museum’s collection of spacecraft and aircraft fifteen minutes away in Chantilly. The shuttle departs every 45-60 minutes 10AM-5:30PM daily, costing 50¢. (A taxi to the museum runs about $15).
Baltimore-Washington International Thurgood Marshall Airport (BWI) is 30 miles northeast of D.C. and 10 miles south of downtown Baltimore, near Glen Burnie, Maryland. It is clearly the farthest-flung, but also offers the nicest in-airport experience.